1. Field of the Invention
The present disclosure relates to a technique for enhancing the reliability of a lighting circuit by determining the light on/off state of a discharge lamp robustly, using a detection signal of a voltage applied to the discharge lamp and that of a current flowing through the discharge lamp.
2. Description of the Related Art
A known discharge lamp lighting circuit employed in an automotive head lamp device or the like comprises a DC-DC converter, a DC-AC converter circuit, or a so-called inverter, and an activating circuit, or a so-called starter circuit. Such a lighting circuit further comprises a circuit for detecting a voltage applied to a discharge lamp and a current flowing through the discharge lamp (for example, see Japanese Patent Document JP-A-10-312896).
Detection values of the voltage and the current of the discharge lamp are used for controlling the power of the discharge lamp, and also, for example, it is used for a light on/off state determination, determining whether the discharge lamp is switched on or off.
For example, one method of detecting the current of the discharge lamp is to detect the current as a value, which is converted to a voltage, by providing a detection resistor, such as a shunt resistor, between the DC-DC converter and the DC-AC converter circuit. One method of detecting the voltage of the discharge lamp is to detect the voltage by voltage-dividing resistors, when the output voltage of the DC-DC converter is substantially equal to the voltage applied to the discharge lamp.
In conventional light on/off determination, it is possible to detect whether the discharge lamp is switched on or off, by using a voltage detection value or a current detection value of a discharge lamp. The voltage detection value or the current detection value is detected, and are compared to the threshold values respectively.
There addressed a problem in that, in the case where the conventional methods of a voltage or a current detection and a light on/off determination cannot be applied, correct determination is hardly conducted.
For example, in a configuration where a power is supplied to a discharge lamp without using a DC-DC converter, by simultaneously conducting a boost of a DC power supply input and conversion from a DC to an AC, the difference, which is the margin, between the detection values when the discharge lamp is switched on and off being small for the detection of the voltage or the current of the discharge lamp, it is difficult to ensure a sufficient detection accuracy. In other words, when the detection value in light-on state is close to that in light-off state, the detection values are hardly distinguished from each other, thereby causing a possibility of error in light on/off determination.